In the use of many optical instruments it is necessary to determine the precise axial location, i.e. the focus, of an image. In the case of photographic cameras, for example, it is necessary to align the photographic film along the image plane. The image plane is in turn located behind the camera lens by a distance which varies as a function of the distance between the object being imaged and the lens.
The human eye, unaided, is generally unable to locate an aerial image with precision due to the automatic accommodation of the human eye. In order to overcome this difficulty, a focusing screen is usually inserted at the expected image plane for the purpose of diffusing images which are not in focus by a sufficient amount so as to overcome the eyes' accommodation.
Focusing screens are generally formed of ground glass which is prepared by surface treatment of a glass sheet usually by an abrasive substance. Ground glass screens involve a number of drawbacks, however. The principal drawback is that the amount of light which passes through the focusing screen decreases as the extent of the blur provided by the screen increases. Ground glass thus cannot provide outstanding focus discrimination without undesirable sacrifice in brightness.
Another disadvantage of ground glass focusing screens is that the surface structure of the ground glass, being relatively large, tends to be visible when used with high magnification eye pieces, resulting in disturbance of the image.